Thistle, Nodding

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Pest Information
Common Name: Thistle, Nodding
Family Name: Asteraceae (Sunflower Family)
Latin Name: Carduus nutans L.
Other Names: Musk Thistle, Nodding Plumeless Thistle, Buck Thistle
Provincial Designation: Prohibited noxious
Seed Act Designation: Primary noxious
Life Cycle: Biennial
Mode of Spread: Seed

Detailed Information
Provincial Situation: Nodding thistle occurs in all Canadian provinces, except PEI. Alberta has sporadic populations mainly in the south.
Description: Carduus nutans can reach the height of 20-200 cm tall, with a long fleshy taproot. Stem is erect, single or up to seven, usually much branched, with spiny wing like appendages. Basal rosette is well developed, leaves are elliptic to lanceolate, 15-30 cm long, glabrous to densely pubescent, pinnately lobed, each lobe ending in a spine. Stem leaves are similar but smaller, simple, alternate and curve down at the edges. Heads are few, terminal and solitary on branches or branchlets. Stem below flower lacks wing like appendages for 1.5-5 cm below the head, or those bearing late maturing flower heads often have a leaf below. Flower head is 1.5-4.5 cm in diameter, usually nodding at maturity; bracts are numerous, glabrous to densely pubescent, overlap, 9-27 mm long, spreading, tapering to a spine 2-4 mm long. Flowers are perfect, tubular, pink to purple or occasionally white; corolla tube 10-14 mm long, 1.4 mm wide. Anthers are 5-8 mm, filaments woolly. Seeds are 3.5-4 mm long, brown at maturity, outer seeds curved.
Two subspecies of C. nutans occur in Canada: ssp. nutans and ssp. leiophyllus.
Key Characteristics:  Interrupted spiny wings along stem except just below the flowering head
 Leaves are dark green, generally with little or no pubescence and deeply lobed with small but sharp spines. The subspecies macrocephalus does have pubescent leaves.
 Silvery leaf margins
 Flower nods when mature
Similar Species: Carduus species can be easily distinguished from species of Cirsium by their filamentous
rather than feathery pappus and, to a lesser degree, by their winged stems. can be distinguished from musk thistle by the fine bristles that are attached to the seeds. Musk thistle has unbranched bristles whereas the thistles in the genus Cirsium have feathery or plume-like bristles.
Plumeless thistle (Carduus acanthoides)
-has smaller flowers (less than 2.5 cm broad) with flower stalks spiny-margined right to the base of the flower head.
Bull thistle (Cirsium vulgare)
 -narrow, spine-tipped bracts.
Marsh Plume Thistle (Cirsium palustre)
-strongly spiny-winged and are usually unbranched except the upper portion which terminates in clusters of purple flower heads
Where it Grows (Habitat & Ecology): Infestations of this weed are limited to pastures, rangeland, rights-of-way, and waste areas, areas where there are tendencies towards overgrazing and in many areas where there is soil disturbances. The wide geographical range covered by C. nutans in North America suggests
that taxa does not have specific climatic requirements.
How it Spreads (Mode of Spread): Nodding thistle reproduces by seed only. Carduus nutans overwinter either as seeds or
  Seed sare dispersed in pappus with the wind, as well as water,wildlife, and livestock when they stick to fur.
Economic Importance (Beneficial Aspects): Nodding thistle has little reports of beneficial aspects. Produces nectar for domestic bees.
Toxicity and Other Concerns: Carduus nutans readily invades disturbed areas and out competes native plants. It is believed to be “alleopathic” slowing the growth of other vegetation near it.
Origin: Native to Europe and Asia. First Canadian specimen of nodding thistle was collected in NB in 1871. Nodding thistle was first found in Alberta in 1976.
Seed Production: Germination of C. nutans seed occurs in the spring and fall approximately 14-21 d after the seed is shed.  Flowering occurs in June-July in Canada. Flowering time of C. nutans in Canada is relatively short  at 3-4 weeks. Seed maturation and dispersal occurs very shortly after flowering 1-3 weeks. Seed production in Canada has been recorded as 165 - 256 seeds per head for C. nutans. Achenes are mainly dispersed by wind and fall near the parent plant (within 50 m) with less than l% being carried further than 100 m. Seed viability remains high over several years,
as trials obtained little loss of seed viability after a 10-yr burial of C. nutans. Musk thistle is a prolific seed producer Average productivity is approximately
10,000 seeds/plant, however, a single plant can produce up to 100,000 seeds (Beck 1999).
Nodding Thistle can produce as many as 20,000 seeds per plant in one season.
Prevention: Prevent the establishment of new infestations by minimizing disturbance and seed dispersal, eliminating seed production and maintaining healthy native communities.
Cultural Control: Hand chopping at ground level just before flowering, or cutting and bagging seed heads
before dispersal can be used to eliminate seed production.
Musk thistle favors abandoned fields and overgrazed pastures. Rotationally grazed or deferred pasture is less susceptible to infestation than heavily grazed pastures. Although some investigators report thistle populations declined rapidly as natural succession proceeded, others report populations that persisted for twelve years in an unmanaged area.
Competition: When compared with six other thistles species, C. nutans seemed to have the lowest competitive ability.
Fertility: Reece and Wilson (1983) reported that the addition of fertilizer enhances the competitive
ability of C. nutans.
Cultivation: C. nutans cannot tolerate regular cultivation.
Mowing: Mowing of C. nutans as a means of control is most effective when applied to flowering plants (McCarty and Hatting 1975). Plants mowed within 2 days of anthesis did not produce viable seeds while those mowed 6 days after produced a significant quantity of viable seeds and those mowed after 11 days produced an amount of viable seed similar to unmowed plants. More than a single mowing per season was required for effective control because of the large differences in plant maturity in natural populations. When plants were mowed before anthesis, regrowth occurred and viable seeds were produced.
Grazing: Infestations may reduce productivity of pastureland and rangeland, by suppressing growth of
desirable vegetation and preventing livestock from eating plants growing in the vicinity of
the thistle.
Fire: Fire has not been effective as a method for directly controlling musk thistle. It is difficult to generate enough heat to kill the thistle's root crown and fire-scarred plants can bolt, flower and fruit.
Bio-Control: 1. Seed Head weevil (Rhinocyllus Conicus) is very effective once established as a colony.
2. Rosette Bud Weevil (Trichosirocalus horridus) is effective.
3. Seed Head Gall Fly (Urophora Solstitalis) is effective.
4. Painted Lady butterfly (Vanessa Cardui)  Larvae defoliate thistles but only migrate  through Alberta  twice a decade.
Chemical Control: Carduus nutans are most susceptible to hormone-like herbicides applied during periods of active growth of the seedlings or rosettes, i.e., during the spring or fall.
Other Canadian Links:
Life Cycle: A biennial, or occasionally winter annual, and rarely an annual.

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Last Modified: January 06, 2012

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